When multiple predator species are introduced to control a single prey species, predator species may act synergistically if the rate of mortality of the prey exceeds the rate of mortality from all single predator species combined. Alternatively, the release of multiple predator species may yield a lower than expected prey mortality due to competitive interactions (including intraguild predation) amongst predators. The question of whether and when multiple natural enemies interact to produce lower or higher prey mortality than each predator species acting alone depends on the details of predator–predator interactions. In this study, we investigated whether combined releases of Phytoseiulus persimilis and Neoseiulus californicus provide better biological control of Tetranychus cinnabarinus on strawberry than releases of each predator species alone. Experiments were conducted in two commercial strawberry fields (Fragaria ananassa Duch. cv. ‘Camarosa’). The release of P. persimilis alone and the combined release of N. californicus and P. persimilis reduced the numbers of T. cinnabarinus on strawberry equally. The densities of T. cinnabarinus in the treatment with N. californicus alone were significantly higher than in the other treatments, and this was the only treatment in which the economic injury level was surpassed. The densities of P. persimilis in absence of N. californicus were similar to those in presence of N. californicus. Hence, there was no significant adverse effect of the presence of N. californicus on the densities of P. persimilis. The densities of N. californicus in the absence of P. persimilis were lower than the densities in the presence of P. persimilis, but this difference was not significant. We conclude that, compared to releases with P. persimilis alone, there is no advantage to releasing N. californicus, either alone or in combination with P. persimilis.